1. Field of the Invention
The present invention relates to a control system for controlling the setting of a shift range in an automatic transmission for a vehicle.
2. Related Art
The automatic transmission for a vehicle is given a fundamental function to set a gear stage automatically on the basis of a running condition such as a vehicle speed or a throttle opening. However, the selection of the shift range, in which the engine braking is effected at a predetermined medium or lower gear stage or in which the gear stage to be set is limited to a predetermined medium or lower one, is manually made by the driver. In the prior art, the selection of that shift range is made by operating a shift lever which is located on the floor or steering column. For this operation, the driver has to leave one hand from the steering wheel or has to turn the eyes temporarily from the front road. This raises operational problems to be improved.
In the invention disclosed in Japanese Patent Laid-Open No. 196118/1993 (or JP-A-5-196118), therefore, the steering handle is equipped with a switch, which is suitably turned ON/OFF to switch the shift range.
The shift range, as can be selected during a forward run, is one for limiting the highest gear stage to be set and for effecting the engine braking at a medium or low gear stage. According to the invention disclosed in the aforementioned Laid-Open, each time the switch is turned ON for an up-range or down-range, an instruction signal is outputted to switch the shift range to one higher or lower by one. Let it be assumed that a downshift occurs when the shift range is switched to a lower one, for example. Then, a frictional engagement element for the engine braking is applied to raise the engine RPM so that an overrun or over-revolution of the engine is caused if the vehicle runs at a high speed. Thus, this switching of the shift range is inhibited from the standpoint of protecting the engine.
Specifically, the switching of the shift range in the automatic transmission is not always made each time an electric instruction is made on the basis of the switching operation. On the basis of the running state of the vehicle, the instruction of the switching of the shift range may be rejected to leave the shift range switching unexecuted. In this case, the switch for switching the shift range in the control system is constructed to output its signal each time it is turned ON. If this instruction signal is rejected, the operation for switching the shift range has to be executed again to increase the number of operations for switching the shift range. Despite of the advantage that the shift range switching can be executed by operating switch, this operation may become troublesome if the instruction of the gear change is rejected according to the state of the vehicle.